1. Field of the Invention
The present invention relates to an image readout lens used for reducing or magnifying an image in an optical system of an image readout apparatus such as image scanner; and, in particular, to an image readout lens with a simple configuration composed of three lens elements.
2. Description of the Prior Art
In a color image readout optical system, employed in an image scanner or the like, of a type adapted to form a reduced or magnified image of an original onto an imaging device such as CCD, a three-element lens has conventionally been known as one which attains a smaller size and lower cost with a simple configuration.
Recently, as demands for this three-element lens to reduce its cost and weight have been increasing, an image readout lens using a plastic as its lens material has been known to fulfill these demands. Though a plastic lens is mainly advantageous in terms of its light weight and low cost as well as its easiness to be processed into an aspheric surface, favorable performances are hard to maintain due to its strong temperature dependence and large deviations of imaging positions upon changes in temperature.
Though conventional examples of a three-element image readout lens are disclosed in Japanese Unexamined Patent Publication Nos. 4-288512 and 4-296808, for instance, they are still insufficient in terms of resolution.